Lyme borreliosis is now the most common arthropod-borne disease in the United States. Borrelia burgdorferi, the causative agent, has been isolated from humans, mammals, birds and arthropods and is cultivable in the laboratory. Careful structural analysis of the DNA content of several early passage isolates has revealed, in addition to a 1,000 kb linear genome, a unique mixture of terminally cross-linked linear and covalently-closed, circular DNA molecules ranging in size from 2 to 50 kilobases in length. Often DNA patterns appear to differ among isolates and to vary during laboratory passage. Given the number of molecules in this apparent extrachromosomal pool, the presence of genes for major surface proteins located on relatively small linear DNA molecules, the uniqueness of the DNA among isolates, and the structural features of these molecules, we have asked if this pool of molecules might not function as a collection of minichromosomes. This view has been supported recently by the description of a similarly segmented arrangement of DNA molecules in B. duttonii - an agent of relapsing fever. The objective of this project, therefore, is to define the genetic capacity of Borrelia burgdorferi in sufficient detail to begin the process of mapping genes and gene products that are important in the pathobiology of this microorganism.